Achieve optimal heat recovery in a kettle exchanger

Hydroprocessing units such as hydrotreaters and
hydrocrackers, and other units such as vacuum distillation towers, visbreakers,
fluid catalytic crackers, etc., involve considerable heat
consumption and subsequent heat recovery. Heat recovery is
mainly achieved with reactor effluents, fractionator
pumparounds or overhead condensing vapors, depending on a
plants process flowsheet. Invariably, this heat is used
to preheat feed or generate steam. Some recoveries take place
in a kettle-type heat exchanger.

Kettle exchangers are mainly used for boiling in the shell
sidee.g., steam generation. Normally, cold liquid
entering the exchanger is close to its boiling point with
respect to the given fluid pressure. The hot fluid flowing in
the tube side may be two-phase overhead condensing vapor,
reactor effluent, single-phase fractionator bottoms or bottoms
from the fractionator pumparound.

For medium-pressure (MP) steam generation, boiler feedwater
(BFW) from the deaerator is preheated by heat from available
intermediate or final product streams. Preheated BFW close to
its boiling point enters the kettle where MP steam is
generated.

However, in some revamp cases, BFW available at the site is
directly allocated in the kettle for MP steam generation. In
such cases, the kettle is expected to handle a sensible heat
load that is more than 10% to 15% of the total heat duty. This
situation leads to an uneconomical design, resulting in
ineffective heat transfer and fouling. To understand this
phenomenon, it is helpful to have an understanding of boiling
basics.

Basics of boiling in the kettle.

Kettles are unbaffled heat exchangers. The tubes are
supported by full baffles. The tube bundle is submerged below a
pool of liquid, and nucleate boiling phenomena normally occur.
In a kettle, liquid generally enters through the bottom of the
shell. Heat is transferred in nucleate boiling through the
combined effect of liquid-free convection and additional
convection produced by the rising stream of bubbles.
Liquid-free convection occurs due to the density difference in
the liquid pool.

In MP steam generation, BFW entering at its boiling point is
further heated by coming into contact with the hot tube
surface, and becomes lighter. The lighter water stream rises
across the bundle between the tube pitch. With greater heat
intake, bubbles are generated on nucleation sites on the tube
surface. These bubbles enlarge and then disengage and rise
above the liquid. Hence, across the tube bundle, the warm water
gradually rises up along with the bubbles. Meanwhile, around
the periphery of the tube bundle, dense, cooler water settles
below. The rising bubbles and the density difference help
circulate the pool. The circulation across the bundle in a
kettle is shown in Fig. 1.1

Fig. 1. Liquid
circulation in the pool.

If the BFW entering the pool is too far below its boiling
point, a sensible heat duty is needed to attain the boiling
point. Thus, the sensible heat duty requirement affects the
boiling process adversely.

This adverse effect can be visualized by imagining a pot of
water heated over an open fire. If a glass top is placed over
the opening of the pot, the boiling process can be easily
observed. As the inner heating surface becomes hot, tiny
bubbles gradually form and stick to the inner surface of the
container. As heating progresses, the bubbles grow and detach
from the inner surface, and then rise to the surface of the
liquid. At this point, even if a small amount of cooler water
is added to the pot, the active boiling process is temporarily
suppressed. Similarly, the boiling process in the kettle is
affected when BFW enters the pool at a lower temperature than
its boiling point.

Boiling heat transfer coefficient in the kettle.

In the shell side, the sensible duty required to heat the
liquid to its boiling point is enhanced by guiding the liquid
through baffles, enabling a velocity increase and a longer
contact time. However, these results are not possible in the
kettle, since it is unbaffled.

Keeping this in mind, the boiling heat transfer coefficient
correlation considers a correction factor that is related to
the ratio of sensible duty and total duty. The presence of
sensible duty is essentially penalized. Thus, the smaller the
sensible duty, the better the overall heat transfer rate of the
kettle.

Effective recovery option for sensible duty.

If a considerable amount of sensible duty is unavoidable,
then an effective option is to add a tiny baffled exchanger at
the kettle entry nozzle, as shown in Fig. 2. At first, BFW is
heated in this baffled shell-side exchanger by the leftover
duty of the hot fluid exiting the kettle. This tiny baffled
exchanger acts as a relatively cold shell for the kettle.

Fig. 2. Schematic of
option for handling
sensible duty.

With this modification, 25% to 30% of the heat-transfer area
can be reduced, compared to a kettle-only design. The
circulation of boiling fluid improves, and the stagnation of
cooler fluid below the shell bundle is minimized. As a result,
fouling is also reduced.

Superheat handling.

Sometimes, hot fluids may have a higher degree of superheat
in the condenser. In such cases, the small exchanger could be
placed above the kettle. The BFW will heat up in the shell side
of the tiny baffled exchanger due to superheated vapor in the
tube side.

After preheating BFW in the baffled exchanger, hot fluid
from the tube side enters the kettle to generate steam. Here,
the tiny baffled exchanger becomes the hot shell. During design
optimization, it can be further assessed whether this small
baffled exchanger at the entry of the kettle is placed as a hot
shell above the kettle or a cold shell below the kettle.

Takeaway.

The accumulation of a large amount of the sensible duty of
the cold fluid should be avoided in a kettle exchanger. If
accumulation is unavoidable, then the sensible duty is
preferably recovered in a baffled heat exchanger.
HP

Tandra Das
has designed and provided support and analysis for heat
exchangers as a thermal engineer at Fluor, KTI and ABB
Lummus Heat Transfer. She also developed engineering
packages as a process engineer at Aker Kvaerner. She
earned an M.Tech degree in process engineering and design
from the Indian Institute of Technology in Delhi, and a BS
degree in chemical engineering from the National
Institute of Technology in
Rourkela.

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I found a problem in factory if hot fluid coming at the top of kettle such as tube leak. I think this is probably because of low vapor pressure at the top of tube causing tube vibration and then, leakage. Therefore, I would like to change the direction of hot fluid to be coming at the bottom instead. Could you recommend us this is a good way or not?